Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (NHCO) and gaseous ammonia (NH3), which is then used to convert NOx. However, in such systems, urea injection levels have to be very precisely controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip. In a typical urea-based SCR catalyst system, the amount of urea injected is in proportion to the feedgas NOx concentration that represents a trade-off between maximum NOx conversion and minimum ammonia slip.
The inventors herein have recognized a disadvantage with the prior art approach. Namely, this approach requires multiple calibration maps to account for complex behavior of the catalyst with respect to adsorption and desorption of ammonia as a function of engine operating conditions and catalyst deterioration, and is therefore inherently inaccurate.
The inventors herein have also recognized that while NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia, it is not necessary that all of the catalyst storage capacity be utilized by ammonia in order to achieve optimal NOx conversion efficiency. Further, the inventors have recognized that, under certain operating conditions, such as at high SCR catalyst temperatures, if the amount of ammonia stored in the catalyst is too high, some of it may desorb and slip from the catalyst or be oxidized to NOx thereby reducing the overall NOX conversion efficiency. Therefore, the inventors have determined that in order to achieve optimal NOx reduction and minimize ammonia slip in a urea-based SCR catalyst, it is crucial to control the amount of ammonia stored in the SCR catalyst. Accordingly, since a direct measurement is not possible, the inventors have developed an accurate method to estimate the amount of ammonia stored in the SCR catalyst.
In accordance with the present invention, a method for estimating an amount of reductant stored in an exhaust gas aftertreatment device coupled downstream of an internal combustion engine, includes: injecting reductant into the device to react with a component of an engine exhaust gas mixture; and estimating the amount of reductant stored in the device based on an amount of said exhaust gas component downstream of the device.
In one embodiment of the present invention, the method is used to estimate an amount of ammonia stored in an SCR catalyst based on an amount of NOx in the exhaust gas downstream of the SCR catalyst.
In another embodiment of the present invention, the amount of ammonia stored is represented by the ratio of the number of the catalyst storage sites containing adsorbed ammonia to the total number of catalyst storage sites normally available, i.e., the ammonia surface coverage fraction of the SCR catalyst.
In another embodiment of the present invention, the method includes predicting the amount of ammonia exiting the catalyst as slip based on the estimated amount of stored ammonia.
In yet another embodiment of the present invention, a method for controlling injection of a reductant into a substance to be reduced by such reductant, such reduction being facilitated by a catalyst, includes: estimating an amount of reductant stored in the catalyst based on an amount of an unreduced substance downstream of the catalyst; and adjusting an amount of reductant injected into the substance based on said estimate.
In yet another embodiment of the present invention, the method includes adjusting the amount of ammonia injected into the SCR catalyst based on the estimate of the amount of stored ammonia.
An advantage of the present invention is that an accurate estimate of the amount of ammonia stored in the SCR catalyst is obtained. Therefore, improved NOx conversion efficiency can be achieved by maintaining an optimum ammonia storage amounts in the SCR catalyst.
Another advantage of the present invention is the ability to accurately predict the ammonia slip, since there are no ammonia sensors currently available commercially.
Yet another advantage of the present invention is that tailpipe ammonia slip is minimized. Additionally, since the downstream NOx sensor is cross sensitive to ammonia, minimizing the ammonia slip improves the accuracy of the NOx sensor readings, and thus improves overall system NOx conversion efficiency.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.